The present invention relates to a series regulator for outputting a constant voltage, and more particularly, to a series regulator circuit switchable between different modes such as a low current consumption mode and a high current consumption mode.
A series regulator circuit is known in the prior art as a circuit that outputs constant voltage even when the input voltage changes. There is a type of series regulator circuit that switches modes for different current consumptions in accordance with, for example, whether a device to which the series regulator circuit is applied is in an operational state or a standby state (refer to, for example, Japanese Laid-Open Patent Publication No. 2001-117650 (FIG. 2) and Japanese Laid-Open Patent Publication No. 2002-312043 (FIG. 1)).
Japanese Laid-Open Patent Publication No. 2001-117650 describes a series regulator circuit including a first constant voltage circuit, which consumes a large amount of current but eliminates ripples and has superior load transition response, and a second constant voltage circuit, which has a low ripple elimination rate and low load transition response but consumes a small amount of current. The series regulator circuit generates an output by switching the constant voltage circuits while using the same output transistor.
Japanese Laid-Open Patent Publication No. 2002-312043 describes a series regulator circuit including a reference voltage generation circuit, which generates a reference voltage, a detection circuit, which generates and outputs voltage that is in accordance with a detected output voltage, a first operational amplifier, which consumes a large amount of current but operates at high speeds, and a second operational amplifier, which suppresses current consumption. The first and second operational amplifiers compare the reference voltage with voltage generated by the detection circuit and provides the control terminal of a transistor with an output corresponding to the comparison result so that the output voltage becomes constant.
Glitches (noise) may be generated when switching currents. Accordingly, Japanese Laid-Open Patent Publication No. 2005-190381 (refer to FIG. 1) proposes a series regulator circuit that suppresses the generation of glitches when the current consumption is switched to a different state.
Japanese Laid-Open Patent Publication No. 2005-190381 describes a constant voltage power supply including two types of constant voltage circuits having different transition responses and current consumptions. When the load state is switched, operational amplifiers are operated in each of the two constant voltage circuits. The generation of noise is suppressed when switching the two constant voltage circuits by providing a period during which an intermittent circuit is activated in each of the two constant voltage circuits.
The series regulator circuits of the above patent publications are formed so that two circuits are switched in accordance with two different current consumption states. Thus, each series regulator circuit includes two operational amplifiers. If the same operational amplifier can be used commonly for the two current consumption states, this would further reduce current consumption of the series regulator circuit. However, the use of the same operational amplifier may lower the response speed, generate glitches when switching modes, and cause output voltage fluctuation such that constant voltage cannot be supplied.